The present invention relates to a swash-plate type compressor and, more particularly, to an engagement structure of a piston and a shoe which are included in the swash-plate type compressor.
Conventional swash-plate type compressors are disclosed in each of Japanese Patent Application Laid-Open Nos. 135990/1986, 65509/1974, and 138474/1981. Each of the swash-plate type compressors comprises a rotatable shaft driven to cause a rotation thereof, a swash plate connected to the rotatable shaft to rotate together with the rotatable shaft, a piston having a concave part facing the swash plate, and a shoe having a spherical convex part inserted in the concave part. When the rotatable shaft rotates, the shoe slides along the swash plate and converts the rotation of the rotatable shaft to a reciprocating movement of the piston in cooperation with the awash plate.
Among these conventional swash-plate type compressors, the compressor using a double-head piston is called a double swash-plate type compressor. The compressor using a piston provided with a compression head only on one end is called a single swash-plate type compressor. In the single swash-plate type compressor, the concave part of the piston is usually made along a single spherical surface having a radius of curvature which is substantially equal to that of the spherical convex part.
During operation of each of the conventional swash-plate type compressors, the shoe performs wobbling movement thereof in the concave part in accordance with the rotating movement of the swash plate. Therefore, the maintaining of excellent lubricating properties is demanded between the spherical convex part of the shoe and the concave part of the piston.
However, the swash-plate type compressor in previous technique has disadvantages in that lubricant oil cannot easily enter between the concave part of the piston and the spherical convex part of the shoe. This is because the concave part of the piston substantially coincides with the spherical convex part of the shoe, there is little clearance between both sliding surfaces.
Therefore, a large force acts on the concave part of the piston, thereby generating wear by sliding movement with the shoe. Particularly in the initial stage of operation of the swash-plate type compressor, since the concave part does not sufficiently fit with the spherical convex surface of the shoe, wear amount is considerable. Moreover, an abnormal wear is sometimes generated in the concave part of the piston.
To prevent this, it is described, for example, in U.S. Pat. No. 4,734,014 that the convex curved surface of the shoe is formed into a spherical surface having a smaller curvature radius than that of the spherical surface of the concave part and that the vertex of the spherical curved surface is formed into a flat part. In the compressor described in this U.S. patent, an oil reservoir is formed between the flat part of the shoe and the accomodation concave part, which is advantage for the lubricating properties.
However, when the end of the flat surface of the shoe slides on the inner surface of the concave part, the local wear of the concave part is more promoted. Therefore, a clearance is generated in a part different from the clearance disposed beforehand. In addition, the load of the local force causes partial deformation in the concave part. So that, the life of the compressor is shortened. Additionally, as a result of reinforcement of relative vibration of the shoe, the generation of noise during the operation is also promoted.